This invention relates to the operation and safety of water-cooled nuclear reactors, and in particular to methods to prevent the deposition of radioactive cobalt and/or inhibit intergranular stress corrosion cracking in the water-bearing vessels of such reactors.
A major hazard in water-cooled nuclear reactors is the accumulation of radioactive substances in the structural portions of the reactor. During reactor shutdown, for instance, workers are exposed to internal walls and tubing surfaces, and radioactive materials retained in oxide films which have accumulated on these surfaces are a major source of radiation exposure.
A second problem encountered in the operation of water-cooled nuclear reactors is referred to as intergranular stress corrosion cracking (IGSCC) which can occur in the weld heat-affected zones of the piping and other system components. Although generally not a safety problem, such cracking is responsible for reduced plant reliability and availability. In cases of severe IGSCC, it has sometimes been necessary to replace entire piping systems at very high costs.
The introduction of certain metallic ions, including zinc, has been used to remove or lessen deposition of radioactive substances. The form and manner in which the zinc has been added, however, has had certain disadvantages. For instances, zinc has been added as a salt with anions which have been detrimental to the reactor, and the use of the active metallic ions before startup or during cleaning shutdowns has been of limited effectiveness in situations where long-term operation has occurred.